The increasing popularity of microwave ovens for cooking all or a part of a meal has led to the development of a large variety of food products capable of being cooked in a microwave oven directly in the food packaging in which they are stored. The convenience of being able to cook food without removing it from the package appeals to a great many consumers. Unfortunately, however, currently available packaging for microwavable food products suffers from some significant disadvantages. A major disadvantage is the inability of this packaging to control the amount of microwave energy received by different areas of the food contained within the packaging. Microwave interactive material may be used in the packaging to promote surface browning and crisping of the food. However, because substantially the same amount of microwave energy reaches the entire food item through the packaging, the thinner areas may be dried out and overcooked while the thicker areas may be barely cooked at all. Frozen food products, such as sandwiches, pastries and the like, which have a thick center section and thinner edges are particularly likely to cook unevenly in available freezer-to-microwave oven packaging.
This type of microwavable food package is described by Turpin et al in U.S. Pat. No. 4,190,757, which includes a microwave interactive layer supported on or adjacent to one of the inside container walls for browning the food in the container. The microwave interactive layer described in this patent, however, suffers from the disadvantages discussed above. Moreover, the heat transferred to the food cooked in packaging containing such a layer may vary over the surface area of the food due to surface or dimensional irregularities and variations in size of the food.
A package assembly for storing and then heating food in a microwave oven is disclosed in U.S. Pat. Nos. 4,555,605; 4,612,431 and 4,742,203 to Brown et al and assigned to the same assignee as the present invention. The packaging assembly described in these patents includes a microwave interactive layer on the bottom of a food tray which is used to form a stand enclosing an air space. The air space is described to promote the even distribution of heat to the underside of the food product. This arrangement functions effectively to brown or crisp food items that have a substantially uniform thickness, such as pizza. However, the application of evenly distributed heat energy to a food that varies in thickness is not likely to brown or crisp all areas of the food to the degree required.
U.S. Pat. No. 4,230,924 to Brastad et al. discloses a food packaging material for microwave cooking that converts some of the microwave energy to heat energy to brown the outside of the food and allows the remainder to dielectrically heat the interior of the food. This packaging material is in the form of a transparent flexible dielectric substrate that has been metallized through a mask so that the coating is subdivided into metallic islands separated by dielectric gaps. This flexible material is intended to be wrapped around and conform to the shape of the food product and is disclosed to affect the degree to which the outer surface of the food product browns during microwave cooking. However, the microwave interactive food wrap described in this patent does not provide the desired control over the degree of browning and crispness of microwave cooked food products, and its use, moreover, is limited to those foods like fish sticks that can be wrapped during microwave cooking.
U.S. Pat. No. 4,258,086 Beall discloses the production of a flexible metallized film useful for wrapping food to be browned in a microwave oven. A patterned metal foil master is employed in conjunction with microwave energy to remove portions of the metallic film coating and create an arrangement of metallic islands separated by dielectric gaps substantially identical to that disclosed in the Brastad et al U.S. Pat. No. 4,230,924. Consequently, the Beall microwave wrap suffers from similar disadvantages. Moreover, neither of these patents suggests that the amount of microwave interactive material left on the metallized food wrap affects or has any relationship whatever to the degree of browning or crisping produced in the food cooked in such wrap.
The prior art, therefore, has failed to provide a food packaging material useful for the microwave heating of a wide variety of foods and food products which employs a microwave interactive material that has been selectively deactivated according to a predetermined pattern to focus the heat generated by the microwave interactive material, thus producing varied temperatures on different surfaces of the food as required to brown or crisp the food properly.